Abrasion- and Peel-Resistant Sol-Gel Coating on Aluminum and Method for Producing a Coating of This Type

ABSTRACT

Provided herein is an aluminum surface having an abrasion- and peel-resistant sol-gel coating. To obtain the coating, the aluminum surface is treated with an aqueous, oxidizing solution, such that said surface provides a suitable base for the subsequent sol-gel coating.

The present invention relates to an abrasion- and peel-resistant sol-gel coating on aluminum surfaces and a method for its production. Furthermore, the present invention relates to a method for the surface treatment of aluminum, in order to improve the adhesion of subsequently applied sol-gel coatings.

PROBLEM

Sol-gel coats impart high corrosion resistance and temperature resistance to metal surfaces. Furthermore, they improve cleanability and protect the surfaces from fingerprints and soiling. Sol-gel coats therefore offer significant economic and qualitative advantages.

Sol-gel coatings on surfaces of stainless steel, steel, copper and copper alloys normally have sufficiently high peel resistance, which permits a deformation of coated surfaces, for example by bending or edging, without the coating coming off and prevents removal by rubbing or wiping.

In contrast to this, sol-gel coatings on surfaces of aluminum and aluminum alloys exhibit a much lower peel resistance. Sol-gel coats applied to aluminum surfaces directly and without special pretreatment of the surfaces become cracked and come off even with slight deformation of the surfaces by bending, even with large bending radii. They can be partially abraded using a thumb or fingernail.

The low peel resistance is caused by the properties of the natural oxide layers, which form due to the influence of oxygen from the environment, and which protect the aluminum from corrosion.

PRIOR ART

Surfaces on aluminum and aluminum alloys to which serviceable and peel-resistant sol-gel coatings are to be applied are subjected according to the prior art to a pretreatment by anodizing before the sol-gel coating. In this, the natural oxide layer is replaced by a modified oxide layer with appropriate suitability as a base for sol-gel coatings. The workpieces to be treated are immersed in a special electrolyte and, using direct current, connected as an anode in a circuit, wherein a layer of aluminum oxide is applied to the surfaces (e.g. DE 10 2012 019 969 A1). These oxide layers are suitable as a base for the subsequent sol-gel coating.

This electrochemical pretreatment by means of anodizing is laborious and expensive. It is limited in application by the shape and size of the workpieces to be treated and calls for special facilities similar to galvanic installations with all the conditions associated with these in regard to environmental protection and wastewater treatment.

Methods for providing a base for a subsequent sol-gel coating on an aluminum surface are mentioned on several occasions in the prior art. However, the aluminum surface regularly has to be freed in the first instance from a naturally present or originally present oxide layer. For example, WO 01/59179 A1 discloses a method for the surface treatment of, among other things, aluminum in a two-stage chemical process combined with mechanical abrasion. In this, the surface is first treated with sulfuric acid in such a way that a fresh, i.e. oxide-free, metal surface is produced. This means that following the treatment of the surface with an acid, fresh oxide layers are formed on the metal in an oxidative process. U.S. Pat. No. 5,356,492 A describes a non-toxic method for treating aluminum surfaces in which the corrosion resistance can be improved by subsequent treatment with an alkaline solution containing molybdenum, nitrite and metasilicate ions. The layer thus obtained can then be improved in respect of its corrosion resistance by applying a sol-gel coat. No information can be gathered from this document as to whether the adhesion of the sol-gel coating to the aluminum surface can be improved by the treatment with hydrogen peroxide, for example, in a multistage process, especially if the sol-gel coating is applied directly following the treatment with the oxidizing agent.

EP 1 457 267 A1 describes a method for manufacturing formed aluminum sheet metal parts with a decorative surface. In the method, described as a coil coating method, the steps are indicated consecutively: provision of a strip of aluminum, optional continuous degreasing of the strip, optional electrochemical, chemical or mechanical burnishing of the optionally degreased strip, continuous pretreatment of the optionally degreased and/or burnished strip to produce a pretreatment layer suitable as a base for a coating layer. Even in this method, any oxide layer that may be present on the metal surface is completely removed before carrying out the process (cf. page 3, line 14). In the oxidative method described, anodic oxidation of the aluminum is regularly carried out. This method guarantees the provision of a porous, anodically produced oxide layer. Aqueous oxidizing agents, wherein the aqueous oxidizing agent has a pH value in a range from 4 to 8, are not proposed in this document.

US 2004/0177898 A1 describes a method for treating metallic surfaces, such as aluminum surfaces, for example. In this method, an oxidizing agent is used together with a sulfate providing agent to obtain a passivated surface, which is to consist substantially of manganese, oxygen, sulfur and carbon. Whether the surfaces obtained in the process described are suitable as a base for a subsequent sol-gel coating cannot be gathered from this document. According to the exemplary embodiments of this document (see in particular example 1), it is provided, however, that when executing the method a pretreatment first takes place, in which any oxides present are completely removed.

The present invention goes back to the realization that aluminum surfaces which already have a naturally formed oxide layer can be pretreated using an aqueous oxidizing solution such that the adhesion of sol-gel coatings to aluminum surfaces pretreated in this way is considerably improved compared with non-pretreated surfaces.

The invention is explained in greater detail in the claims.

The subject matter of the invention is an aluminum surface provided with a sol-gel coating, which surface was obtained in that, in a first step, a pretreatment of the surface already provided with an original or naturally formed oxide layer was carried out using an aqueous oxidizing solution, followed by drying and subsequent application and hardening of a sol-gel coat.

In the case of the present invention, original or natural oxide layer is understood to mean an oxide layer that normally forms directly on aluminum surfaces as soon as their surface was treated in mechanical or other ways so that a bright aluminum layer is obtained. On such a bright aluminum layer a so-called passive layer is formed, which consists substantially of aluminum oxide. Such a natural aluminum oxide layer is not normally adequate as a base (see also the examples in the experimental section of this application).

According to the invention, abrasion- and peel-resistant sol-gel coatings are obtained on aluminum surfaces that usually have a layer thickness of 0.5 to 5.0 μm. These coatings can also be colored. Inorganic color pigments are especially suitable for this.

Inasmuch as aluminum surfaces are spoken of in the present application, aluminum alloys are also addressed by this, the chemical characteristics of which are comparable to an aluminum surface.

The subject matter of the invention is in particular a method for the treatment of aluminum surfaces, which are acted upon by an aqueous, oxidizing solution. Investigations have revealed that in the treatment of oxide layers naturally present on aluminum surfaces with an oxidizing, aqueous medium, these layers are modified in such a way that they are suitable as a base for sol-gel coatings to be applied subsequently.

The aqueous oxidizing agents contain normal oxidizing agents, such as peroxides, persulfates, perchlorates, perborates in a suitable concentration. Furthermore, oxidizing agents such as potassium persulfate or hydrogen peroxide are also suitable.

Other constituents are not necessary in the aqueous solution, but are not generally to be necessarily excluded either.

The aqueous oxidizing agents have a pH value in a range from 3 to 8, or 4 to 8. The concentration of hydrogen peroxide, for example, is normally in a range from 1 to 30 percent by weight. Solid oxidizing agents are usually added in a quantity of 5 to 50 g/l (for example, 5 to 50 g/l potassium persulfate).

The treatment of the surfaces normally takes place at room temperature by immersion, spraying or wiping over a period of at least 2 minutes, preferably of 5 to 10 minutes.

It has turned out, however, that a longer treatment period is not necessarily disadvantageous.

Following the treatment of the aluminum surface with the aqueous oxidizing agent, the surfaces are rinsed with water, for example, and then dried.

Following the drying of the surfaces, the sol-gel coating is applied.

Sol-gel coatings are normally obtained from two reaction components, which are mixed shortly before processing in a fixed ratio to one another. Finally, as a third component, a thinner, usually an alcohol, is added to this mixture. The concentration of the reaction mixture and the viscosity of the finished formulation are adjusted by the thinner.

It is clear to the person skilled in the art that the sol-gel is applied first in the form of a liquid sol with colloidal particles floating therein, which is then transformed into a gel and following thermal hardening finally forms a solid, hard covering layer. When “application of the sol-gel coating” and “thermal hardening of the sol-gel coating” are thus spoken of, the person skilled in the art knows the state which the sol-gel system is in.

The sol-gel is preferably a silica sol, based on silanes, which are dissolved in solvent, wherein the silica sol preferably also contains one or more other sol-forming elements, preferably one or more elements from the group consisting of Al, Ti, Zr, Mg, Ca and Zn, wherein these elements replace the Si atoms in the colloidal structures. Preferred sol-gel coatings/sol-gel lacquers are described in EP2145980. Reference is made hereby in particular to the sol-gel coatings described in EP2145980 and the method for their application.

The starting compounds for forming the preferred sols and ultimately the sol-gel coating are preferably hydrolysable silanes of the formula SiR₄, wherein the 4 residues comprise R 2-4 hydrolysable residues OR′ and 0-2 non-hydrolysable residues R″. These starting silanes can thus also be represented as Si(OR′)_(4−n)R″_(n) with n=0.1 or 2. If additional sol-forming elements such as just described are used, corresponding compounds according to the valences of the elements should be selected as starting compounds, such as AIR₃, etc.

The hydrolysable residues OR′ are hydroxy, alkoxy and/or cycloalkoxy residues. Suitable examples of these comprise, for instance, hydroxy, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, i-butoxy, t-butoxy, pentoxy, hexoxy, cyclopentyloxy, cyclohexyloxy residues, wherein in particular ethoxy, n-propoxy and isopropoxy residues are preferred. The hydrolysable residues OR' can be identical or different from one another.

The non-hydrolysable residues R″, such as are present, are alkyl and/or cycloalkyl s residues. Suitable examples of these comprise, for instance, methyl, ethyl, n-propyl, isopropyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, cyclopentyl, cyclohexyl residues, wherein in particular methyl, ethyl, n-propyl and isopropyl residues are preferred. The non-hydrolysable residues R″ can likewise be identical or different from one another.

The starting compounds of the preferred sols can consist of a single type of silane, but will frequently comprise mixtures of a plurality of silanes (and where appropriate additional sol-forming starting compounds of other elements). It is preferred that at least one of the components of the starting compounds is a silane of the formula Si(OR′)_(4−n)R″_(n) with n=0, thus Si(OR′)₄. For example, a preferred sol-gel lacquer can is comprise the starting materials TEOS (tetraethoxyorthosilane) and MTES (methyltriethoxysilane), and/or DMDES (di methyldiethoxysilane).

Apart from these, other common additives in the field of sol-gel systems can naturally also be used, for example additional network-forming components, such as acryloxypropyltrimethoxysilane and methacryloxypropyltrimethoxysilane, which can ensure further organic crosslinking, in particular if a not insignificant portion of the starting compounds are so-called network-changing compounds of the formula Si(OR′)_(4−n)R″_(n) with n=1 or 2.

In the sol, the starting compounds are partially hydrolyzed to the corresponding hydroxy compounds (for instance, ortho silicic acid, trihydroxyalkylsilane etc.), which process can be promoted by the addition of a catalyst, for instance of acid. Due to the high tendency of these hydroxy compounds to condensation, these can now condense with splitting of water into smaller siloxane networks. Colloidal particles are already present in the sol, which particles contain siloxane bonds. Siloxane bonds are bonds of the form ≡Si—O—Si≡, wherein “≡” symbolizes three optional bonds, which are independent of one another, with other elements, in particular with OH, OR′ and R″, whereby a three-dimensional crosslinked structure is created in the colloidal particles. Here OR′ and R″ have the same meaning as above.

The sol-gel coating normally has a baking temperature of below 300° C., preferably of 200° C. to 250° C.

The sol-gel coating can also be colored, for example by adding inorganic color pigments. Due to the degree of coverage with inorganic color pigments, i.e. due to the weight proportion of the inorganic color pigments in the sol-gel, the color intensity and depth can be adjusted.

The viscosity of the sol-gel coat can be adjusted by the person skilled in the art. It is known that with a suitably high thinning in its solvent, the sol is sufficiently thin to be applied by spraying, misting, rolling or painting.

Suitable solvents for the sol are water and above all alcohols such as methanol, ethanol, n-propanol or isopropanol, wherein ethanol and isopropanol are preferred on account of their physical properties and the low toxicity of their vapors.

The sol-gel can contain inorganic color pigments, e.g. SICOCER® Black 10901, SICOCER® Blue 2502 or SICOCER® Red 2355 from BASF.

The sol-gel coating is preferably applied by spraying or rolling, misting or painting on are also possible. However, it is preferably sprayed, as this permits precise control of the quantity applied per unit of area.

Following the coating, the surfaces can be dried until the solvent has evaporated. The dried surfaces are then thermally hardened. Preferably no discoloration of the coating occurs during the hardening. The thermal hardening in step (III) takes place preferably at a temperature of below 300° C., preferably in a range from 200° C. to 300° C. The hardening preferably takes place for a period of approx. 20 to 60, preferably of 30 minutes at temperatures in the range from 160° C. to 280° C., preferably 200° C. to 250° C. in air.

The glass ceramic coating preferably has a thickness of 0.5-5.0 μm, preferably 1.0-5.0 μm, or 0.5-3.0 μm and at most preferably 1.0-3.0 μm. The glass ceramic coating preferably has a uniform thickness with variations of preferably less than 10% of the coat thickness.

According to the invention, an aluminum surface is achieved with an abrasion- and peel-resistant sol-gel coating, which is distinguished by improved abrasion resistance and adhesion. A particular advantage of the invention here lies in the fact that an abrasion- and peel-resistant surface is obtained in a particularly simple, low-cost and environmentally friendly manner. Another advantage lies in the fact that compared with the prior art, limitations in respect of the shape and size of the workpieces to be treated are scarcely to be taken into consideration in the method according to the invention.

EXAMPLES

In the following examples, various aluminum surfaces with a bright surface of AlMg 4.5 were first degreased, rinsed and dried. A natural oxide layer had already formed previously on these sheets before the surface was treated further and provided with a sol-gel coating.

Example 1

Two aluminum sheets of AlMg 4.5 with a surface polished with grain 240 were degreased, rinsed, dried and treated as follows:

A sol-gel coating was applied by spraying to one sheet without further pretreatment and was then baked for a period of 30 minutes at 240° C. in air. After cooling, the sheet was bent around a radius of 10 d by 90°. The sol-gel coat became cracked in the process and flaked off in the area of the bending.

The second sheet was immersed for a period of 10 minutes in an aqueous solution with 10% by weight hydrogen peroxide, then rinsed with water and dried. Then an identical sol-gel coating was applied by spraying and baked at a temperature of 240° C. for a period of 30 minutes in air. After cooling the sheet was bent with a radius of 2 d by 180°. The coating remained free of cracks, homogeneous and firmly adhering in the area of the bending.

Example 2

Two aluminum sheets of AlMg 4.5 with a bright surface were degreased, rinsed, dried and treated as follows:

A sol-gel coating was applied by spraying to one sheet without further pretreatment and was then baked for a period of 30 minutes at 240° C. in air. After cooling, the sheet was bent with a radius of 10 d around an angle of 90°. The sol-gel coat became cracked in the process and flaked off in the area of the bending.

The second sheet was sprayed for a period of 10 minutes with an aqueous solution with 10% by weight, then rinsed and dried. Then an identical sol-gel coating was applied by spraying and baked at a temperature of 240° C. in air. After cooling the sheet was bent with a radius of 2 d around an angle of 180°. The coating remained free of cracks, homogeneous and firmly adhering in the area of the bending.

Example 3

Two aluminum sheets of AlMg 4.5 with a bright surface were degreased, rinsed and dried.

A sol-gel coating was applied by spraying to one sheet without further pretreatment and was then baked for a period of 30 minutes at 240° C. in air. After cooling, the sheet was bent with a radius of 10 d around an angle of 90°. The sol-gel coat became cracked and flaked off in the area of the bending.

The second sheet was sprayed for a period of 5 minutes with an aqueous solution containing 40 g/l of potassium persulfate at pH 4, then rinsed and dried. Then an identical sol-gel coating was applied and baked at a temperature of 240° C. in air. After cooling the sheet was bent with a radius of 2 d around an angle of 180°. The coating remained free of cracks, homogeneous and firmly adhering in the area of the bending. 

1. A method for providing a base for a directly following sol-gel coating on an aluminum surface with a natural oxide layer, comprising treating the aluminum surface with an aqueous oxidizing agent, selected from the group consisting of peroxides, persulfates, perchlorates and perborates, wherein a pH value of the aqueous oxidizing agent is in a range of 3 to 8, and wherein the oxidizing agent is used at a concentration of 5 to 50 g/l.
 2. The method according to claim 1, wherein the treatment with the aqueous oxidizing agent is carried out at ambient temperature for a period of at least 2 minutes.
 3. The method according to claim 2, wherein the ambient temperature is 15° C. to 25° C.
 4. The method according to claim 1, wherein the treatment with the aqueous oxidizing agent is carried out over a period of 5 to 10 minutes.
 5. (canceled)
 6. A method for producing an abrasion-resistant aluminum surface, comprising: a) pretreating an aluminum surface by the method according to claim 1: b) rinsing the pretreated surface and then optionally drying the surface; and subsequently c) applying a sol-gel coat and hardening the sol-gel coat.
 7. The method according to claim 6, wherein the sol-gel coating on the aluminum surface has a coat thickness of 0.5 to 5.0 μm.
 8. An aluminum surface with an abrasion- and peel-resistant sol-gel coating obtained according to a method according to claim
 6. 